1. Field of the Invention
The present invention relates to coatings for paper or other sheets or substrates, which are generally classified in the art as release coatings and are characterized by their ability to separate intact from a surface or material which is normally adherent or adhesive in nature. There are many applications for such release coatings ranging from packaging materials for tacky products, such as asphalt, to release papers for such uses as preparing heat and pressure consolidated decorative laminates.
2. Description of the Prior Art
The requirements for release coatings and release sheets may vary widely, depending upon the application in which they are to be employed. For example, a release sheet which is adequate for use as a packaging material may not satisfy the requirements necessary for a release sheet used in the casting of a thermoplastic film or sheet; and, similarly, a release sheet which is adequate for a casting application may not be suitable in a platen press application wherein high temperatures and/or pressures are employed to consolidate decorative laminates. This latter application places particularly demanding requirements on the release sheets.
Decorative laminates have found widespread acceptance as marresistant surfaces for wall-coverings, paneling, table tops, counter tops, vanities, and the like. These laminates are conventionally formed by consolidating several laminae under heat and pressure to form a unitary structure which usually carries on at least one of its surfaces a surface decoration which can range from something as simple as a solid color to something as complex as an embossed, simulated woodgrain finish.
Preparation of such laminates generally involves the use of one or more sheets of corestock in combination with a decorative or print sheet and, if desired, a top sheet or overlay. The corestock, if corestock is employed, usually comprises an unbleached kraft paper which has been impregnated with a relatively inexpensive thermosetting resin such as a phenolic resin, a polyester resin, and the like, which is easily crosslinked upon the application of heat and pressure during consolidation.
The decorative or print sheet and the overlay (if used) are both impregnated with a "noble" thermosetting resin (i.e. a resin which is also cross-linkable upon the application of the heat and pressure during consolidation, but which exhibits little or no color deterioration upon the application of such heat and pressure and which prevents any strikethrough or "bleeding" of the thermosetting resin used in the corestock). Two of the most common "noble" thermosetting resins used to impregnate the decorative or print sheet employed in the preparation of such laminates are urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates, although other resins such as polyester resins have also been employed as "noble" thermosetting resins for the impregnation of such sheets.
After consolidation, in those applications where the resulting laminate is intended to be used in a structural application, rather than merely for its decorative effect, the laminate is usually bonded to a substrate material such as plywood, hardboard, particle board, cementasbestos board, and the like, to give it additional strength and rigidity for its intended structural use.
As indicated above, the various laminae employed to form the ultimate laminate are consolidated through the simultaneous application of heat and pressure between heated pressure platens. Such presses generally consist of two heavy, heated platens on the top and bottom of the press, with additional heated platens in between to provide the number of openings desired in the press. Each opening typically includes the following elements in the following order: (a) a heated platen, (b) one or more sheets of caul stock, which are primarily intended to distribute pressure uniformly over the surface of the caul plate, (c) a metal caul plate, (d) the opening itself, (e) another metal caul plate, (f) one or more sheets of caul stock, and (g) another heated platen. The various laminae which will form a laminate are placed together in the opening, sometimes alone and sometimes as a "book" or "pack" of laminates.
In order to prevent the decorative sheet or overlay sheet from adhering to the caul plate, and in some cases to also impart desired gloss or surface texture to the decorative sheet or overlay sheet, it is customary to employ a release sheet or liner between the print sheet or overlay sheet and the caul plate. The laminae material which is placed against the release sheet is typically an uncured amine formaldehyde resin, such as melamine formaldehyde resin condensate or urea formaldehyde resin condensate. Such materials have a strong tendency to react with and/or adhere to the surface of the release sheet, placing tough requirements upon the release sheet, which must resist adhering to the laminate. This difficult requirement is further complicated by the fact that different manufacturers use different laminate materials and different press temperatures and pressures. Furthermore, the variation in heating and cooling cycles occurring from laminate to laminate and edge to center in each laminate, even in a single pressing operation can result in variable release results.
A particularly satisfactory prior art release coating and release sheet for use with platen pressing of decorative laminates is disclosed in U.S. Pat. No. 3,946,135, issued to John O. H. Peterson on Mar. 23, 1976. This patent, which is herein incorporated by reference, discloses a release coating formulation comprising a release agent, a monomeric polyhydric alcohol and a cross-linkable thermosetting resin. The monomeric polyhydric alcohol cross-links with the cross-linkable thermosetting resin to form a particularly stable surface having a low level of reactance with and/or adherence to laminates used in panel pressing. However, the release sheets coated from the above-described formula are sometimes not entirely satisfactory from a release standpoint for use with all varieties of laminate materials and press conditions. Also, the release coating formulation, which is coated as an aqueous solution onto a paper substrate, is sometimes difficult to coat because of its low viscosity. Attempts to increase the viscosity by adding conventional thickener materials have generally been unsatisfactory because of their tendency to rapidly cross-link with the other materials in the formulation to increase the coating viscosity very quickly beyond that which is coatable, resulting in an unacceptably short pot life.